Sintered ceramic materials have been used, in various industrial sectors, as electronic and structural materials for heavy electrical components, aircraft components, automobile components, electronic equipment components, precision machinery components, semiconductor equipment and the like. This is due to the fact that, in general, the sintered ceramic materials are less likely to decline in strength even at high temperatures than and moreover are superior to metallic materials in common use in respect of a large number of physical characteristics such as hardness, electrical resistance, abrasion resistance, heat resistance, corrosion resistance, weight saving and the like.
The sintered ceramic material noted above, however, is deficient in that owing to its smaller tensile stress than its compression stress, it is brittle as usually termed so and hence is liable to get instantaneously broken under rather small a tensile stress. For that reason, it has been strongly demanded that sintered ceramics be enhanced as regards its toughness and breaking energy with a view to rendering ceramic components applicable to those parts requiring high reliability.
Namely, ceramic structural parts for use as components of gas turbines, aircraft, automobiles and the like should be resistant to heat and strong at elevated temperature and besides high in reliability. In this respect, researches are being made at institutes at home and abroad in an effort to put to practical use a composite ceramic material which is designed to afford toughness, breaking energy and other properties at a high level by addition of a reinforcement such as an inorganic material- or metal-induced fiber, a whisker, a plate, a powder or the like to a sintered matrix in compositely dispersed manner.
For instance, certain ceramic-based composite fiber materials have been developed which are obtained with use of a ceramic matrix that is formed by bundling 500-3000 long fibers of ceramics, each fiber of about 10 .mu.m in diameter, to thereby prepare a fiber bundle (yarn), and by orienting a plurality of fiber bundles in a two- or three-dimensional direction, or by weaving them together into a preform of a given shape (fiber preform), followed by formation of a matrix in the preform as by a CVI method (Chemical Vapor Infiltration method). Alternatively, a ceramic matrix is derived by filling particulate ceramics in the above preform by means of cast molding and subsequently by sintering the resultant molding to form a matrix in which fibrous ceramics are thereafter disposed in composite condition.
Some of the ceramic-based fiber composite materials discussed above are sufficiently effective in increasing fracture toughness and fracture energy and are highly conducive to improved reliability insofar as they are in the form of a specimen on a relatively small scale. However, such a fiber composite material has the problem that when in actual use for large-sized components, particularly for thick-walled components, it causes a sharp decline in strength and fracture energy. Another problem is that the fiber composite material is not easily applicable to components of a complicated shape. Still another problem is that the above fiber composite material has monofilaments or fiber bundles oriented simply in one direction and hence tends to become anisotropic with respect to its material characteristics, and this means that the fiber composite material fails to adequately cope with a wide variety of product shapes.
Where the above conventional methods are employed to form a matrix, it is virtually difficult to fill the matrix interiorly of a fiber bundle or a fiber preform in uniform and dense condition. When a matrix is formed for example by the CVI method previously mentioned, the filling ratio of a matrix has been found to be in the order of 70-90% at most irrespective of the case with a fiber bundle or a fiber preform, and a matrix can only be obtained with more voids in the preparation of thick-walled components. Extended matrix formation is further involved so that production efficiency is extremely reduced.
On the other hand, matrix formation resulting from cast molding of powder ceramics enables a matrix to be filled in a fiber preform except for a fiber bundle in a ratio of about 90%-100%. The ceramics, however, causes reduced filling ratio when filling in a minute fiber bundle constituted with a plurality of fibers each of about 10 .mu.m in diameter. Thus, the resultant composite material as a whole invites irregularly varied ratio of matrix filling and entails low initial cracking strength of the matrix and small fracture energy of the composite material after crack initiation, eventually failing to gain great reliability.
As noted above, the ceramic-based fiber composite materials of the prior art have been molded to date into various trial products but at a small or specimen level. There is left the problem that no methods have been yet established as to the production of large-sized components, especially thick-walled equivalents, from those composite materials. Consequently, it is difficult to shape such a composite material as desired and also to control the same microstructurally with the result that those characteristics typified by strength, toughness and the like are still unfeasible to a full and stable extent.
In addition, no definite methods of evaluating the characteristics of composite ceramic materials per se have been established, nor have any standards been provided which could be relied upon in detailed material design as well as product design and component design. These circumstances pose an obstacle to the progress and practice of composite ceramic materials.
The present invention has been made to eliminate the foregoing problems. A first object of the invention is to provide a ceramic-based fiber composite material which exhibits great reliability and high toughness by way of uniform formation of a constituent matrix to enhance initial matrix cracking strength and fracture energy.
A second object of the present invention provides a method of the production of a ceramic-based fiber composite material that is adequately adaptable to various product shapes and also excellent particularly both in strength at elevated temperature and in toughness, which method comprises controlling such composite material microstructurally to afford these characteristics.